An accelerometer is a very common measurement device found in a multitude of systems. In the automotive industry, for instance, acceleration sensing is commonly used for airbag deployment. The computer industry utilizes accelerometers to protect hard disks from large shocks, and the aerospace industry employs inertial measurement units comprising multiple accelerometers and gyroscopes for sensing and navigation. Accelerometers are currently found in many personal handheld devices as well, where they are used to detect the general orientation of the device. In many high volume applications, the majority of accelerometer devices are made using microelectromechanical systems (MEMS) fabrication technologies.
These techniques allow for the devices to be batch fabricated in a CMOS process flow, enabling dramatic reductions in size, weight, power, and cost (SWaP-C) while maintaining adequate performance for a variety of applications. Examples of such techniques may be found in [1] Seshia, et al., “A Vacuum Packaged Surface Micromachined Resonant Accelerometer,”, JMEMS, Sys., Vol. 11, No. 6, (2002), [2] R. Hopkins, et al., “The silicon oscillating accelerometer: a high-performance MEMS accelerometer for precision navigation and strategic guidance applications,” ION NTM 2005, San Diego, Calif., pp. 970-979, (2005) and [3] A. Trusov, et al., “Silicon Accelerometer With Differential Frequency Modulation And Continuous Self-Calibration,” 2013 IEEE MEMS Conf., Taipei, Taiwan, pp. 29-32, (2013).
FIG. 1 shows an example of such a MEMS-based accelerometer. The resonant frequencies of two vibrating sensing tethers 102 are detected and used to calculate the acceleration experienced by a large proof-mass 104, onto which the tethers are attached. In standard resonant MEMS accelerometers, the two vibrating sensing tethers 102 are excited and detected using electro-static comb drives 106. These comb drives 106 can be used to both excite motion in the tether 102, typically at its natural mechanical resonant frequency, as well as to detect this induced motion.
The purpose of the comb-drive detection is to precisely measure the resonant frequency of the tethers, acting as a small strain gauge. The tethers are attached to the large proof mass 104, which experiences displacement as a result of applied acceleration. This proof mass displacement pulls one of the tethers 102 into tension while pushing the other tether 102 into compression, altering the resonant frequencies of the tethers 102 as a result. The resonant frequency shifts have equal magnitudes, but opposite sign, only if the proof mass acceleration occurs in the desired axis. Any acceleration, and resulting displacement, experienced in orthogonal dimensions, forces the tether resonant frequencies to shift together, which allows for a differential measurement and a cancellation of unwanted signals.
For inertial navigation applications, however, there is a general desire to improve upon the sensitivity of accelerometers, while simultaneously improving the stability of the measured signals over long time periods.
The parameter associated with resonant accelerometer sensitivity is the scale factor, which is the amount of frequency shift experienced by an individual tether as a result of a given acceleration of the proof mass attached thereto. The scale factor may be expressed in units of Hz/g. A larger scale factor is generally desirable, not only to increase system sensitivity, but also to reduce the impact of unwanted drifts in the sensor signal due to temperature and other fluctuations in the surrounding environment over time. For example, in the case where the scale factor is equal to 10 Hz/g and the tether resonant frequency is stable to within 1 Hz over long periods of time, the measured signal, in units of measured acceleration, will drift by 0.1 g over this time. If instead the scale factor is increased to 10 kHz/g (a factor of 1000) and the tether frequency stability stays exactly the same, the measured signal will now drift by only 0.1 mg (a factor of 1/1000). This scale factor is dependent on the ratio of the size of the proof mass to the size of the tether, where larger proof masses and smaller tethers result in larger scale factors.
Since acceleration measurements need to be integrated twice to retrieve position, measurement errors and/or noise in the original signal can produce significant errors in final assumed position. Consequently, there is a large effort to improve performance of these devices to reduce this measurement error. To date, accelerometers with improved performance typically come at the expense of size and power, moving away from MEMS fabrication technologies to take advantage of a larger proof-mass in order to achieve higher sensitivity and long term stability. There is a need to break this trade-off and develop accelerometers with excellent sensitivity and long-term stability, while maintaining the low SWaP-C of MEMS devices.